This section provides background information related to the present disclosure which is not necessarily prior art.
High-energy density electrochemical cells, such as lithium ion batteries, can be used in a variety of consumer products and vehicles. Typical lithium ion batteries comprise a first electrode, such as a cathode (positive electrode), a second electrode such as an anode (negative electrode), an electrolyte material, and a separator. Often a stack of lithium ion battery cells is electrically connected to increase overall output. Conventional lithium ion batteries operate by reversibly passing lithium ions between the negative electrode and the positive electrode. A separator and an electrolyte are disposed between the negative and positive electrodes. The electrolyte is suitable for conducting lithium ions and may be in solid or liquid form. Lithium ions move from a positive electrode to a negative electrode during charging of the battery, and in the opposite direction when discharging the battery. Each of the negative and positive electrodes within a stack is connected to a current collector (typically a metal, such as copper for the anode and aluminum for the cathode). During battery usage, the current collectors associated with the two electrodes are connected by an external circuit that allows current generated by electrons to pass between the electrodes to compensate for transport of lithium ions.
Many different materials may be used to create these components for a lithium ion battery. By way of non-limiting example, cathode materials for lithium batteries typically comprise an electroactive material which can be intercalated with lithium ions, such as lithium-transition metal oxides or mixed oxides of the spinel type, for example LiCoO2, LiMn2O4, LiNiO2, LiNi(1−x−y)CoxMyO2 (where 0<x<1, y<1, and M may be Al, Mn, or the like), or lithium metal (e.g., iron) phosphates. The electrolyte typically contains one or more lithium salts, which may be dissolved and ionized in a non-aqueous solvent. The negative electrode typically includes a lithium insertion material or an alloy host material. Typical electroactive materials for forming an anode include lithium-graphite intercalation compounds, lithium-silicon intercalation compounds, lithium alloys and lithium titanate Li4+xTi5O12, where 0≦x≦3, such as Li4Ti5O12 (LTO), which may be a nano-structured LTO. Contact of the anode and cathode materials with the electrolyte can create an electrical potential between the electrodes. When electron current is generated in an external circuit between the electrodes, the potential is sustained by electrochemical reactions within the cells of the battery.
Li-ion batteries can suffer from capacity fade attributable to many factors, including mechanical degradation from volume expansion during lithium intercalation or from the continuous formation of a passive film, known as the solid electrolyte interface (SEI) layer, over the surface of the negative electrode (anode), which is often generated by reaction products of anode material, electrolyte reduction, and/or lithium ion reduction. The SEI layer formation plays a significant role in determining electrode behavior and properties including cycle life, irreversible capacity loss, high current efficiency, and high rate capabilities, particularly advantageous for power battery and start-stop battery use.
Moreover, certain anode materials, like LTO, have particular advantages, such as high operating voltage relative to a lithium metal reference potential that desirably minimizes or avoids SEI formation. LTO is also a zero-strain material having minimal volumetric change during lithium insertion and deinsertion, thus enabling long term cycling stability, high current efficiency, and high rate capabilities. Such long term cycling stability, high current efficiency, and high rate capabilities are particularly advantageous for power battery and start-stop battery use.
However, while LTO and other materials may be promising anode materials for high power lithium ion batteries, potentially providing extremely long life and exceptional tolerance to overcharge and thermal abuse, in certain circumstances, when used with certain cathode materials and electrolytes, LTO may have certain disadvantages. For example, it has been observed that Li4+xTi5O12 can generate significant quantities of gas, which mainly consists of hydrogen, within a battery cell especially at elevated temperature conditions under charging state. Such gas formation can make it an undesirable choice for commercial use. For safe and successful use, it would be desirable to improve various anode materials, like LTO, either to minimize formation of SEI layers and/or to minimize or suppress gas formation, to provide durable batteries with sustained high capacity, high discharge rates, and long life.